In direct sequence spread spectrum (DSS) communication, a wide band carrier signal is modulated by a narrow band message signal. The wide-band carrier is typically generated by modulating a single frequency carrier using a pseudo-random noise (P/N) code sequence. The data rate at which a message is communicated is usually much lower than the P/N code symbol or “chip” rate. The ability of DSS to suppress interference is proportional to a ratio of the chip rate and the data rate. In many applications, there are thousands of code chips per data bit.
At the receiver, a carrier replica is generated by reducing the DSS signal to baseband and multiplying it with a locally generated replica of the original narrow-band carrier using a local oscillator. If the frequency and phase of the carrier replica is the same as that of the received original narrow-band carrier, then the multiplier output signal will be the product of the bipolar P/N code and intended message. The P/N code is removed by multiplying the wide-band data stream with the locally generated replica of the P/N code that is time-aligned with the received P/N code. This is the de-spreading process.
Generating the carrier replica with proper carrier frequency and phase and generating the P/N code replica at the proper rate and time offset is a complex problem. In many DSS communication systems, the necessary carrier frequency, carrier phase, and P/N code offset are not known a priori at the receiver, which tries different values until a large signal is observed at the data-filter output. This is termed the search or acquisition process, and a DSS signal is said to be acquired when the proper frequency, phase and code offset have been determined. A receiver selects and detects a particular transmitted signal by choosing the appropriate P/N code and performing the acquisition search. In some cases the acquisition search must include examination of different P/N codes from a known list when the transmitting node is not known. When many different codes, code offsets and carrier frequencies must be examined and the SNR is low, the acquisition task can be both time and energy consuming.
The above constraints are more pronounced in a secure environment such as that depicted in FIG. 1 (detailed below), where a new node termed a hailing node 34 seeks to join an existing network while maintaining security for the joining node and those nodes already on the network. In addition, an established network requires a method of discovering the existence of another separate network that may have migrated into communication range, so that a cross-link can be established between the networks in order to form a larger network. The process of nodes “discovering” each other is termed node discovery, and is where DSS signal acquisition occurs. Typically, node discovery is done on channels separate from the primary data communication channels. Limited data exchange on the “discovery channel” is preferable for network optimization. As a result, the discovery waveform must be flexible in the messages it carries and not be constrained to one specific message type or size.
The air interface should consist of a flexible and symmetric full-duplex or half-duplex link. The transmitting node or hailing node is that node that sends a discovery signal, essentially a message inquiring as to the presence of receiving nodes. Receiving nodes are the nodes that listen for the discovery signal. The receiving nodes are therefore target nodes, which may have already formed a network. These receiving nodes may become transmitting nodes when they send an acknowledgment back to the initiating new node. In this way, a new node that flies into range of an established network will transmit discovery messages on that transmitting node's transmit link. When a receiving node in the established network hears the discovery message on the receive link, it will respond via its transmit link which is the hailing node's receiving link. Subsequent handshaking can then be performed via the two nodes' transmit and receive links to bring the initiating new node into the network. The transmitting and receiving links may occupy separate time slots in a time division duplex (TDD) system, or may be separate frequency bands in a frequency division (FDD) system.
An exemplary but non-limiting environment in which node discovery may be important is illustrated in perspective view at FIG. 1, a prior art arrangement of disparate nodes operating in a traffic data network and one hailing node seeking to join the traffic network. The nodes may be airborne as in aircraft; terrestrial as in autos, trucks and trains; or waterborne as in ships and other surface watercraft. They may be stationary or mobile, fast or slow moving, as for example, communications between nodes in a building, an aircraft, and an automobile. For additional flexibility, it is assumed that a hailing node 34 may not have a clock signal synchronized with the network prior to joining. The range 22 of the traffic data network is centered on a command node 24, absent relays by other nodes within the network. Where the network links members via a satellite link, the line-of-sight range 22 is not particularly relevant. The range 22 is included to show further advantages of the invention that may be exploited when network communications are geographically limited.
The command node is representative of the node that receives the discovery signal, and may be a true command node that controls access to the secure network (in that no other nodes receive and acknowledge discovery signals) or it may represent any node already established within the network that receives the discovery signal (such as where all established nodes listen for discovery signals). In FIG. 1, all nodes depicted as within the traffic network range 22 communicate on the traffic network, either through the command node 24 or directly with one another once granted network entry. The traffic network typically operates by directional antennas 24a, at least at the command node 24, to maximize the network range 22. This is because directional antennas typically enable a higher antenna gain and a higher tolerable path loss as compared to omni-directional antennas. Therefore, a range (not shown) of a discovery network that operates using omni-directional antennas 24b is somewhat less, at least in the prior art. The command node 24 maintains communication with stationary nodes 26, 28. When two nodes are aircraft, they may be closing or separating from one another at very high rates, rendering Doppler effects significant. When a hailing node 34 sends a discovery signal to locate and request entry into the traffic network, its signal is typically not received at the command node 24 until the hailing node is within the traffic network range 22. Since the hailing node 34 is not yet identified as authorized, this potentially puts communication within the network at risk, or alternatively unduly delays granting the hailing node 34 access to the network. Because access to the traffic network is obtained through the discovery protocol, that protocol must exhibit security features to prevent compromise of the traffic network.
Considering the issues apparent in light of FIG. 1, a good node discovery scheme for a highly secure communications network would therefore exhibit (a) high speed and reliability; (b) long range; (c) low probability of intercept (LPI) and low probability of detection (LPD) by unauthorized parties; (d) universal discovery and recognition among the various nodes; (e) asynchronous discovery; and (f) reliability for both stationary and fast-moving nodes. Each of these aspects are detailed further at co-owned and co-pending U.S. patent application Ser. No. 10/915,777, herein incorporated by reference in its entirety as if fully restated herein.
Transmission signals are normally divided into preamble and payload sections, payload carrying the substantive data. In a discovery signal of the prior art, the preamble and payload sections were at the same frequency and the receiving node would search among the possible frequency bins until it acquired the signal preamble. This prior art approach has been described as the receiver spinning its frequency search. In Doppler environments where transmitter and receiver may move relative to one another at a rate unknown prior to acquisition, as with the hailing and command nodes of FIG. 1, the frequency at which a discovery signal reaches a receiver is unknown to the extent of Doppler uncertainty. Ensuring the prior art receiver locks onto a discovery signal payload within the very short time of that signal preamble (e.g., on the order of milliseconds) with a high degree of probability requires a large hardware commitment. The present invention uses a different discovery signal regimen to reduce the hardware requirement in the receiver while simultaneously decreasing acquisition time in a highly secure communication environment.
Accordingly, in such communications environments the goals of security, hardware simplicity and speed of operation are often conflicting. For example, security requires very long and often complex spreading codes. Such codes require complicated circuitry on the transmitting and receiving sides which, in addition, require relatively large amounts of power to operate. Further, communications signals spread with such codes may take relatively long to acquire.
As a result, in most if not all situations encountered in such communications environments, fast acquisition of communications channels is desired. Fast acquisition contributes favorably to desired goals of low probability of intercept and resistance to jamming. Fast acquisition, though, may require relatively short and simple spreading codes, negatively impacting security. Those skilled in the art do not desire that speed of acquisition sacrifice security, pointing back to the use of long spreading codes.
For these reasons, those skilled in the art desire methods and apparatus for use in channel acquisition that have security performance usually associated with long spreading codes, while having speed of acquisition usually associated with short codes. Those skilled in the art also desire methods and apparatus for use in channel acquisition that are applicable to frequency hopping spread spectrum communications systems and hybrid time-division or frequency-division spread spectrum communications systems.